1. Field of the Invention
This invention is directed to a novel cement composition for the preparation of a novel aqueous slurry useful in cementing casing in the borehole of a well comprising (1) cement, (2) (a) a hydroxyethylcellulose ether having a critical viscosity or (2) (b) a mixture of a hydroxyethylcellulose ether having a critical viscosity and of a hydroxypropylcellulose ether having a critical viscosity and (3) a dispersant.
2. Description of the Prior Art
After a borehole of an oil or gas well has been drilled, casing is run into the well and is cemented in place by filling the annulus between the borehole wall and the outside of the casing with a cement slurry, which is then permitted to set. The resulting cement provides a sheath surrounding the casing that prevents, or inhibits, communication between the various formations penetrated by the well. In addition to isolating oil, gas and water-producing zones, cement also aids in (1) bonding and supporting the casing, (2) protecting the casing from corrosion, (3) preventing blowouts by quickly forming a seal, (4) protecting the casing from shock loads in drilling deeper and (5) sealing off zones of lost circulation. The usual method of cementing a well is to pump a cement slurry downwardly through the casing, outwardly through the lower end of the casing and then upwardly into the annulus surrounding the casing. The upward displacement of the cement slurry through the annulus can continue until some of the cement slurry returns to the well surface, but in any event will continue past the formations to be isolated.
If the primary cementing of the casing, as described above, does not effectively isolate the formations, it may become necessary to perforate the casing at intervals along its length and then squeeze a cement slurry under high pressure through the perforations and into the defined annulus to plug any channels that may have formed in the cement sheath. Squeezing is an expensive operation that requires bringing perforating and cement service companies back to the well and is therefore to be avoided, if possible.
It is critical in preparing cement compositions useful in cementing casing in the borehole of a well that they be characterized by little or no fluid loss, the presence of little or no measureable free water, a viscosity designed for optimum particle suspension, optimum pumpability, flow properties sufficient to facilitate and maintain laminar and/or plug flow, adequate gel strength to provide thixotropic properties to the slurry when pumping ceases, thickening time tailored or designed to meet field specifications, high compressive strength and substantially no shrinking on setting.